Upgrade node-usage to 0.3.8 to fix memory usage units on OSX

Memory usage was off by a factor of 1024 on OSX in node-usage 0.3.7, which was corrected in 0.3.8.
Merge pull request #73 from jhurliman/patch-1

Upgrade node-usage to 0.3.8 to fix memory usage units on OSX